hello bugbear,
a proposition can be evaluated as being either true or false. So for example:
"your username is bugbear."
This a proposition because it is either true, and your username is bugbear, or it's false and you have some other username.

An example of a sentence which isn't a proposition is:
"What's your username?"
Because this sentence cannot be answered with true or false.

Another example of a sentence which isn't a proposition and therefore isn't a statement is:
"The yellow cat."
A bit like your example this isn't even a proper sentence, because there's no verb.